The invention relates to optical blanks made from polyorganoheterosiloxanes and a method of making the same.
Glass is a material which by virtue of the diversity in its chemical composition allows the achievement of specifically desired optical data the most important of which are the refractive index n.sub.D and the Abbe number v.sub.D. However, there is the drawback that glass must be melted down at very high temperatures requiring much energy and that the optical blank, for example a lens, must be produced by expensive grinding and polishing treatment. Moreover, glasses frequently have high densities and are not fracture resistant without special provisions. These drawbacks are very apparent in spectacle lenses so that glass is being increasingly replaced by plastics materials.
In respect of optical data, plastics have more limitations than glasses. However, they are produced at lower temperatures and have greater fracture resistance. Furthermore, the density of plastics is only about half that of glasses. Moulded blanks made from plastic materials, for example optical lenses, are obtained by application of very time-consuming casting processes using expensive, long time-blocked glass molds or of the more rational injection molding process. However, plastics (thermoplastics) which are produced by application of the latter process scratch very easily and those made by application of the first mentioned process, such as for example CR39 which is predominantly used for spectacle lenses, are somewhat less vulnerable to scratching but still considerably more so than glass.
From EP-A-78 548 hydrophilic polyorganoheterosiloxanes are known which are worked into contact lenses by casting, hardening in the mold and subsequent cutting and other mechanical treatment. However, this method is labor and time consuming.